A noncontact IC card is provided with a resonant circuit. When an antenna coil which is part of the resonant circuit is moved to cross a magnetic flux generated from an information read/write device, an induced electromotive force occurs from the antenna coil and the resonant circuit produces a high voltage. When the noncontact IC card is brought close to an antenna section of the information read/write device and caused to receive electromagnetic waves from the antenna section, the resonant circuit is brought to a state of resonance. Data communication is carried out between the information read/write device and the noncontact IC card using the state of resonance. Also, the voltage produced as a result of resonance of the resonant circuit is used to supply electric power to other circuits causing those circuits to operate.
Let us now consider actual conditions of use of the noncontact IC card. Since noncontact IC cards used as railway commuter passes, cash cards used at banks and credit cards used for shopping, for example, are so thin that a plurality of noncontact IC cards of this kind are held overlaid in a wallet or a pass holder and brought close to an antenna section of an information read/write device in many applications.
Since two or more noncontact IC cards are overlaid with each other when in use in this case, resonant frequencies of the individual noncontact IC cards deviate from carrier frequency of a signal transmitted from the information read/write device due to mutual inductance between the noncontact IC cards. As a result, there can arise a case where the voltage produced in the resonant circuit drops and electric power sufficient for signal reception is not supplied to internal circuits of the noncontact IC card in use, leading to an inability to perform communication between the information read/write device and the noncontact IC card.
This phenomenon is now explained using FIG. 17. The horizontal axis of a graph of FIG. 17 represents the frequency of a resonant circuit of a noncontact IC card. The vertical axis indicates the absolute value of a receiving voltage occurring in the resonant circuit. This is similarly applied to the horizontal axis and the vertical axis of a later-described graph of FIG. 18.
FIG. 17(A) shows a relationship between frequency and receiving voltage that occur when one noncontact IC card is brought close to an information read/write device. In this case, the noncontact IC card is used alone. For this reason, resonant frequency f0 of the resonant circuit matches carrier frequency fc of a carrier signal transmitted from the information read/write device, and a high receiving voltage occurs at that frequency.
FIG. 17(B) shows a relationship between frequency and receiving voltage that occur when two noncontact IC cards are overlaid with each other and brought close to the information read/write device. In this case, the two noncontact IC cards are overlaid and are subject to the influence of mutual inductance between the two noncontact IC cards, so that the resonant frequency of the resonant circuit varies and, as depicted in the Figure, the resonant frequency f0′ deviates from the carrier frequency fc of the carrier signal from the information read/write device. For this reason, a low receiving voltage occurs at the carrier frequency fc as compared to the receiving voltage at the resonant frequency f0′. As a result, there arises a problem that sufficient electric power is not supplied to internal circuits of the noncontact IC card in use, leading to an inability to perform communication between reader/writer and the noncontact IC card.
To solve the aforementioned problems, it has been proposed to preset the resonant frequency of the noncontact IC card such that it deviates to a higher frequency as compared to the carrier frequency. According to this method, the resonant frequency does not deviate so much from the carrier frequency even if the resonant frequency decreases when two noncontact IC cards are overlaid with each other and, therefore, it is possible to supply sufficient electric power to the internal circuits of the noncontact IC card in use.
This method is described referring to FIG. 18. The noncontact IC card is set such that the resonant frequency f0 of its resonant circuit is higher than the carrier frequency fc as shown in FIG. 18(A) when the single noncontact IC card is used.
Shown in FIG. 18(B), on the other hand, is a case where two noncontact IC cards overlaid with each other are used. Since the two noncontact IC cards are used together and the two noncontact IC cards are subject to the influence of mutual inductance, the resonant frequency f0′ becomes lower than the carrier frequency fc.
Regardless of whether the single noncontact IC card is used or the two noncontact IC cards are used together, however, the resonant frequency f0, f0′ does not deviate so much from the carrier frequency fc as compared to the aforementioned case described with reference to FIG. 17. It is therefore possible to decrease the drop of induced electromotive force compared to the case described with reference to FIG. 17.
Another previously proposed method is such that a switch is series-connected to the resonant circuit and a judgment is made to determine whether the noncontact IC card is overlaid with another noncontact IC card by monitoring the voltage of the resonant circuit while turning on and off the switch at random. If it is judged that the noncontact IC cards are overlaid with each other, the switch of one of the noncontact IC cards is turned off and the resonant circuit of the other noncontact IC card brought to a state of resonance. In this method, one noncontact IC card is in the state of resonance while the other noncontact IC card is not in the state of resonance when they are overlaid.
With this arrangement, it is possible to obtain the same receiving voltage as would be obtained when one of the noncontact IC cards is used alone. In addition, the other noncontact IC card can receive energy of the resonant circuit of the first noncontact IC card thanks to electromagnetic coupling caused by mutual inductance. Thus, either of the two noncontact IC cards can obtain the intended voltage (Japanese Laid-open Patent Publication No. 10-126318).
Either of the aforementioned methods, however, has drawbacks stated below:
(1) According to the method of offsetting the resonant frequency of a noncontact IC card from the carrier frequency, the resonant frequency is offset from the carrier frequency even when the single noncontact IC card is used. Thus, the induced electromotive force decreases as much as the amount of frequency offset and the communication range decreases.
(2) According to the method of randomly turning on and off the switch connected to the resonant circuit, it is necessary to keep a receiver section and a clock circuit inside the noncontact IC card operating continuously to detect whether the noncontact IC card is overlaid with another one. For this reason, power consumption of the noncontact IC card increases and it requires a large magnetic field. It is necessary for the noncontact IC card to communicate at a short range in order that it can receive this large magnetic field.
It is an object of the invention to provide noncontact communications media which can communicate when multiple noncontact communications media are overlaid.
It is another object of the invention to provide noncontact communications systems using the aforementioned noncontact communications media.